1. Field of the Invention
This invention relates to a process for preparing a tungstate salt. More particularly, the invention concerns a process for preparing ammonium tungstate and related compounds from reduced iron/manganese tungstates or ores or concentrates thereof.
2. Prior Art
Iron and manganese form a range of tungstate minerals varying in composition between FeWO.sub.4 and MnWO.sub.4. These minerals are generally referred to as ferberite, wolframite and huebnerite, depending on the composition of the mineral. Generally, ferberite is used to describe such a mineral containing more than 20% by weight of FeO and huebnerite when such a mineral contains more than 20% by weight of MnO. The intermediate compositions are referred to as wolframite, although this term is often used for the whole range of compositions; such use may be made hereinafter. These iron and manganese tungstates are important sources of tungsten trioxide and metallic tungsten.
In conventional processes for recovering tungsten values from wolframite ores, the ores are usually concentrated by gravity, magnetic and/or flotation techniques. The concentrates so formed are then processed further. Tungstate salts are often formed as intermediates in these processes. For example, sodium tungstate can be obtained by fusing a concentrate with sodium carbonate or by contacting a concentrate with hot sodium hydroxide solution. Acidification of the aqueous sodium tungstate solution yields insoluble tungstic acid. Alternatively, tungstic acid can be obtained from the concentrate by leaching the concentrate with a strong acid, such as concentrated hydrochloric acid. The tungstic acid can then be dissolved in ammonium hydroxide to form ammonium tungstate, which in turn can be isolated, dried and heated to form tungsten trioxide, tungsten and other useful tungsten compounds by techniques such as those described in K. C. Li and C. Y. Wang, "Tungsten", American Chemical Society Monograph 130, Reinhold Publishing Corporation, New York (1955), especially Chapter IV.
The ammonium tungstate is believed to be the paratungstate salt, which may be defined as having the formula x(NH.sub.4).sub.2 O.yWO.sub.3.zH.sub.2 O with, for example, x=3 and y=7 or x=5 and y=12, the value of z depending on the conditions of crystallization of the salt.
Each of the above-described known processes is useful for extracting tungsten values from various tungsten compositions. However, lower levels of impurities in products obtained in good yields are technically and economically desirable. For example, ammonium tungstate manufactured by known processes is susceptible to contamination by impurities. The concentration of such impurities depends at least in part on the actual process conditions employed in the manufacture of the ammonium tungstate and on the composition of the tungsten ore.